The invention is directed primarily to apparatus in which an electron beam is swept over a target and a narrow or accurate definition is required for the target area illuminated by the beam. An important example of such a device is a scanning electron microscope, in which an electron beam is swept over a target and secondary electrons or back-scattered primary electrons are collected to generate a signal representing the topographical features scanned by the beam. This signal is then suitably displayed, for example on a cathode ray screen, to provide an enlarged picture of the target. The resolution of the system depends in part on the narrowness of the electron beam and the accuracy with which the position of the beam is controlled.
When the target to be scanned has an electrically non-conducting surface, there is a buildup of negative charge on the surface due to the deposition of electrons from the scanning beam. The field associated with this charge diverts the beam and also spreads it somewhat, with a resulting adverse effect on resolution. It also disturbs the trajectories of the low-energy secondary electrons, thereby diverting a substantial portion from the electron detector and thus degrading the output signal. Moreover, the charge buildup can cause random, localized voltage breakdowns at the target surface, with a resulting deleterious effect on the image.
To overcome this problem the target surface is often rendered electrically conductive by coating it with a suitable conducting material. However, this is a time consuming procedure and it often masks some of the very features which one desired to perceive by means of the microscope. It may also prevent the use of concurrent analysis techniques such as X-ray analysis and Auger electron analysis.
The same problem is encountered in electron beam lithography. An image is traced on a target by an electron beam to control a subsequent etching process thereon. The electron beam is used in order to obtain higher resolution than is readily obtainable with the more conventionally used optical illumination. However, if the target is electrically non-conductive, charge buildup prevents attainment of the desired resolution, causes registration errors and results in other deleterious effects.
The problem is also involved with the examination of insulating specimens in scanning Auger microprobes and in electron microprobes. In Auger microprobes the charge buildup both disturbs the incoming primary beam and causes energy shifts in the Auger spectrum. In electron microprobes the primary beam disturbances are the predominant problem.
One proposed solution to this problem is the inclusion of a secondary electron gun in the vacuum chamber. This gun projects a beam of electrons having a relatively low energy (e.g. 1 keV). In this energy region, the beam causes a net discharge of electrons from the target, thereby tending to positively charge it. Ideally, the positive-charging effect of this beam can be stabilized in position and time to neutralize the negative-charging effect of the primary, high-energy beam, thereby limiting charge buildup on the target surface.
This method has several drawbacks, however. The neutralizing beam cannot be operated simultaneously with primary beam, because electrons emitted from the target as a result of impingement of the neutralizing beam will swamp the signal generated by primary-beam impingement. Therefore, the two beams must be turned on and off alternately, line-by-line or point-by-point. Also, the output of the electron detector must be interrupted when the neutralizing beam is on. Further complications result from the need to have the neutralizing beam accurately cover the area swept by primary beam on the target surface. Moreover, neutralization of negative charge buildup by means of electrons, which are themselves negative, is an inherently unstable process and care must be taken to avoid a runaway condition.